This invention pertains to communication signal receivers. More particularly this invention pertains to digital signal processing in receivers.
In the next decade, communication enabled devices of various types are expected to proliferate. Among the communication enabled devices that are expected to proliferate are those employing small, low power, low cost transceivers. Such devices are expected to find use in asset tracking systems, wireless sensor networks, industrial and environmental monitoring, control systems, wireless personal computer peripherals, toys, and security systems among other things. In order to foster their proliferation, designs that lend themselves to reduced cost manufacturing are needed. Furthermore, devices that feature low power consumption and may be operated for long periods of time on small batteries or low power environmental energy sources (e.g., solar cells) are desired.
In recent years radio frequency (RF) receivers that are partly digital and offer low cost, low power consumption and high performance have been proposed. An important element in RF receivers employing digital circuitry is the analog to digital (A/D) converter. Because A/D converters sample signals at discrete points in time, and represent signals amplitudes by a limited number of discrete values, they create a type of noise termed quantization noise. This quantization noise can decrease the performance of a receiver, decreasing its ability to receive signals without error, or increasing the amount of power or spectrum required to transmit signals without error. Thus there is a need to reduce the quantization noise introduced in A/D converters of partially digital receivers. However this should be done without degrading the information carried in the received signals.
The A/D converter defines a partition between analog and RF portions of the receiver circuit and the digital portion of the circuit. A first approach to partitioning receivers is to mix a received signal to base band in the analog domain, then convert the base band signals to digital signals using base band delta-sigma A/D converters. This technique is widely used because of the high dynamic range of the base band delta-sigma converter. While this technique is widely used, it places difficult requirements for noise and linearity on analog circuits to perform the operation of converting the received signal to a base band signal. These requirements can typically be overcome though the use of expensive components and high power dissipation, both of which are undesirable.
A second approach makes use of a band pass delta-sigma converter as the A/D. In this approach, the received signal is converted to an intermediate frequency (IF) signal in the analog domain. The IF signal is then converted to the digital domain using the band pass delta-sigma sampler. Additional down conversion to base band is accomplished in the digital domain. While this approach addresses the noise and linearity issue of base band conversion approaches, the band pass delta-sigma converters tend to be high in power consumption and circuit area (an therefore cost) due to the need for band pass filters in their sampler circuit.
In communication systems that use phase shift key modulation (e.g., QPSK, OQPSK, QAM) each quantum of information (e.g., bit or chip) is represented in a base band signal by a pulse having a predetermined pulse shape (e.g., a one-half cycle Sine wave shaped pulse). In receivers of such systems in order to increase the receivers ability to detect such pulses (i.e., in order to attain high receiver sensitivity) it is desirable that the transfer function of the receiver be characterized by an impulse response function that has a functional form that matches the pulse shape of the quantum of information. In the frequency domain such an impulse function corresponds to a low pass filter with a corner frequency that corresponds to frequency limit of the base band signal.
What is needed is a receiver that includes a digital demodulator in which the effect of quantization noise on the receiver""s sensitivity is controlled.
In particular, what is needed is a receiver that includes a digital demodulator in which the effect of quantization noise on the receiver""s sensitivity is controlled.
What is needed is a digital demodulator that can perform the operations of mixing to base band, decimation and filtering while maintaining high receiver performance and minimizing circuit complexity and cost.